The present invention relates to the art of force measurement, and more particularly to a transducer for generating electrical signals representative of forces applied to a rotating shaft by a passing web in tension.
The transducer of the present invention finds particular utility when mounted between an underlying stationary fixture and an overlying pillow block including a rotational bearing support for an idler roll in contact with a moving web. In such an application, tension in the web produces downward forces on the roll which are transmitted through the pillow block to the transducer causing movement thereof with respect to the stationary fixture. As a result, the transducer produces electrical signals representing the tension in the web.
In a typical web tension measurement system, the web passes over and is held in contact with an idler roll supported at both ends in the bearing of a pillow block. In this configuration, the tension in the moving web transmits a generally downward force through the shaft and the pillow block to the underlying transducer. Each transducer generates one or more electrical signals representative of the applied force for quantification of the magnitude and in some cases the direction thereof. The resulting transducer output signal is commonly used for monitoring and/or control of the tension in the web using, for example, motor speed servo controls and the like. As the tension in the moving web increases, the applied downward force on the shaft and hence the force on the transducer increases proportionally. As a result, the transducer electrical output signal also increases. Similarly, a decrease in the web tension results in a corresponding decrease in the transducer output signal level. Such tension monitoring and control systems are common in manufacturing applications involving continuous feed materials such as cloth, metals, plastic, and paper manufacturing and printing, as well as in conveyor belt systems and the like, where tension is an important parameter. The transducer generally includes strain gage devices mounted either externally on the transducer body""s peripheral surfaces or internally on the inside walls of holes or slots machined in the transducer body. Single or multiple strain gages may be employed in or on the transducer body in various known electrical interconnection configurations such as quarter-bridge, half-bridge, and full-bridge designs. The particular strain gage interconnection configuration is selected based on desired electrical characteristics including the sensitivity which is usually expressed in units of millivolts per volt (mV/V). The strain gage or gages are connected in bridge fashion with a DC excitation power source, typically 10 VDC. A common sensitivity of 2 mV/V for a full-bridge configuration at 1000 microstrains would yield 20 microvolts per microstrain. Certain electrical interconnection configurations, moreover, can be implemented to reduce or cancel the effects of transverse or common mode forces or vibration such as longitudinal or horizontal forces applied to typical web tension transducers.
Heretofore, strain gages in such transducers have been mounted onto the inner walls of slots machined out of the transducer body. The body is typically mounted to a stationary fixture using mounting fasteners at the extreme longitudinal ends of the body with a gap between the lower surface of the body and the fixture. The gap allows the transducer body to flex in response to downward forces transmitted by the pillow block. One or more horizontal slots extend laterally through the body, typically near one or both of the longitudinal ends of the body. Strain gages are mounted to the upper and/or lower horizontal surfaces of the slots to sense the flexing of the upper and lower slot walls which are much thinner than the body itself As downward force is applied to the body, the upper and lower slot walls flex, and the strain gages mounted thereon provide a signal in response from which the amount of force, and hence the web tension, can be determined.
In a typical configuration, two strain gages are mounted longitudinally to the upper slot wall, and two gages are longitudinally mounted to the lower slot wall. The gages are electrically connected in a half-bridge circuit configuration with an external pair of lead wires for excitation power positive and negative connections as well as an output signal connection. An external instrument such as a force monitor or controller provides excitation voltage, typically 10 VDC, to the excitation power lead wires and receives a signal, usually in millivolts, from the signal lead wire. This output signal may then be filtered and/or amplified with the resulting signal being used by other instrumentation to determine the force applied to the mounted transducer body.
As with any measurement transducer, noise immunity in the signal is an important performance feature. This is commonly expressed as the signal to noise ratio S/N, which represents the magnitude of the signal span as compared to the magnitude of the noise. Electrical common and/or normal mode noise from external sources can be superimposed on signal leads, particularly where the instrumentation is remote from the transducer device. The use of shielded twisted pair cables has been successful in reducing such noise. Mechanical common mode noise is also an important problem. Vibration of the transducer body in its longitudinal direction will tend to stretch and compress the strain gages mounted in the longitudinal orientation. Full or half-bridge electrical bridge configurations can effectively cancel such common mode effects where the gages mounted on the upper slot wall are deflected the same amount and in synchronism with those mounted on the lower slot wall. However, quarter-bridge configurations remain susceptible to such mechanical noise disturbances which appear as spurious signals on the transducer output signal lead wires.
The signal to noise ratio can be improved with respect to both mechanical and electrical noise sources by reducing or canceling the noise itself using techniques such as those previously discussed. Heretofore, these efforts have yielded some measure of protection against noise. Alternatively, the signal to noise ratio can also be improved by increasing the level of the strain gage signal, such as by increasing the mechanical deflection of the strain gage per unit of applied force. This can be done by maximizing the distance between the strain gage slot and the pivot points about which the transducer body flexes. In the existing transducers, the gage slots are located at the ends of the body, near the mounting bolts. The moment about the ends of the body is minimal at this location. The corresponding deflection in the strain gages is therefore also minimal. The optimal deflection in existing transducers is at the center of the body where the moment about the end mounting points is maximized. However, there remains a need to provide a web tension transducer with improved signal to noise ratio.
In accordance with the present invention, there is provided a web tension transducer by which the foregoing and other problems and disadvantages are minimized or overcome. More particularly, and in accordance with the principal aspect of the present invention, there is provided a web tension transducer with a body mounted at opposite longitudinal ends to a stationary fixture, which body pivots about a single horizontal axis near one longitudinal end thereof with strain gages mounted in a single slot located at the opposite longitudinal end. In this respect, the strain gages measure the mechanical deflection of the transducer body at a location where the moment is maximum about the horizontal pivot axis. The strain gage output signal is thereby increased with respect to the prior art devices for a given applied force, consequently improving the signal to noise ratio of the device. All the various strain gage configurations previously available are usable with the present invention, as are the previously discussed cabling techniques. By the arrangement of the present invention, therefore, the immunity against both electrical and mechanical noise has been improved by purely mechanical means.
In accordance with another aspect of the invention, the active part of the transducer is made from a single piece of metal which enables a watertight design.
In accordance with another aspect of the invention, the transducer includes a removable adaptor plate allowing use of the transducer with any number of pillow blocks.
In accordance with yet another aspect of the invention, the transducer body top surface is vertically spaced above the longitudinal end mountings to allow pivotal movement thereof in response to applied downward force.
It is accordingly a primary object of the present invention to provide an improved web tension transducer with increased electrical and mechanical noise immunity.
Another object of the present invention is the provision of a transducer of the character described above which allows use of various strain gage configurations in order to achieve improved noise immunity.
Yet another object of the present invention is the provision of a transducer of the character described above that is watertight.
A further object of the present invention is the provision of a transducer of the character described above which allows the use of a single transducer with a number of different pillow blocks by the provision of an appropriate adaptor plate.